follow-up has 3 distinct synonyms spread across 3 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an activity that continues something that has already begun or that repeats something that has already been done
Synonyms, antonyms and the broader and narrower relations for follow-up come from Princeton University’s WordNet. Full sources and licences.
a subsequent examination of a patient for the purpose of monitoring earlier treatment